1. Field of the Invention
The present invention relates to a high-frequency oscillator, and more particularly, to a high-frequency oscillator switching and outputting signals of a plurality of frequencies.
2. Description of the Related Art
A high-frequency oscillator that outputs a 100-MHz or higher frequency by using a crystal resonator with high stability is used as a frequency source, etc. of an optical communication. The high-frequency oscillator normally amplifies the output of a crystal oscillator with frequency multiplication, for example, by using a plurality of LC filter stages and an amplifier, and obtains a target high-frequency output.
In recent years, instead of this configuration, a high-frequency oscillator having a configuration where a high-frequency output is obtained by distorting the output of a crystal oscillator, and by selecting a harmonic component with a SAW (Surface Acoustic Wave) filter has been implemented, for example, as recited in Japanese Patent Application No. 2000-244682. With this configuration, the number of elements is decreased, so that the outer dimensions of the oscillator are significantly reduced.
FIG. 1 is a circuit diagram of the high-frequency oscillator having the above described configuration.
The high-frequency oscillator shown in this figure is configured by a crystal oscillator 1, an SAW (Surface Acoustic Wave) filter 2, and an amplifier 3. The crystal oscillator 1 is composed of a crystal resonator 4 which is, for example, AT-cut, a split capacitor which is not shown and forms a resonant circuit along with the crystal resonator 4, and an oscillation amplifier 5 which feeds back and amplifies a resonant frequency. Here, what is called an oscillation circuit of a colpitts type is formed.
The SAW filter 2 is configured by forming, for example, input/output interdigital transducers (IDTs) on a piezoelectric substrate not shown. The SAW filter 2 makes only a component in a particular frequency region of an input signal pass through, and outputs the component.
The amplifier 3 is implemented as a broadband amplifier 3 such as a linear IC amplifier, etc. With the amplifier 3, the output of the SAW filter 2 is amplified with a linear portion, that is, a non-saturation portion of the input/output characteristic of the broadband amplifier 3 so that a high-frequency output is obtained.
The crystal oscillator 1 referred to here is a voltage control type implemented by inserting a voltage variable capacitance element 6 in an oscillation closed loop of the crystal oscillator 1. The oscillation frequency of the crystal oscillator 1 is varied by a control voltage Vc that is applied via a high-frequency blocking resistor 7. Additionally, Vcc in FIG. 1 is a power source voltage.
In the high-frequency oscillator shown in FIG. 1, the oscillation output of the crystal oscillator 1 is distorted. For example, the top of a sinusoidal wave is cut and distorted to be rectangular as shown in FIG. 2 by making a center voltage Voo of the oscillation output higher than a center voltage Vco of the power souce voltage Vcc. The oscillation output of the crystal oscillator 1 is distorted, so that the levels of harmonic components f2 through fn, relative to a fundamental component f1 within a frequency spectrum, are raised in the oscillation output as shown in FIG. 3.
The respective components of the frequency spectrum are made equal to or higher than a predetermined level as shown in FIG. 3, whereby an arbitrary harmonic component can be selected by inputting this oscillation output to the SAW filter 2. For example, the fundamental frequency (fundamental component) f1 of the crystal oscillator 1 is implemented to be 155.52 MHz, which is almost as high as the manufacturing limit, and a harmonic component 622.08 MHz, which becomes a quadruple-frequency wave f4, is selected with the SAW filter 2, and amplified with the amplifier 3, so that a high-frequency output is obtained. If the oscillation output of the crystal oscillator 1 is not distorted, the level of a harmonic component relative to the fundamental component is low. Therefore, the harmonic component cannot be selected with the SAW filter 2.
However, with the high-frequency oscillator having the above described configuration, only a signal of one high frequency, for example, only a signal of 622.08 MHz can be supplied. Accordingly, if a communications appliance requires signals of 2 frequencies such as the fundamental frequency (155.52 MHz), and the harmonic frequency being a quadruple-frequency wave (622.08 MHz), 2 high-frequency oscillators are mounted on a set substrate, and an output signal from either of the oscillators is selected. Therefore, the configuration of the high-frequency oscillator becomes large in this case, leading to an increase in the cost, and an obstacle to the downsizing of the set substrate.